The present invention relates generally to mask or template printing and, more particularly, to means for determining the dimensions of figures printed on semiconductor wafers.
In the fabrication of integrated circuits, a mask of known dimensional accuracy may typically be employed to project or print a pattern repetitiously onto a semiconductor wafer by photographic, X-ray, or electron beam techniques. It is often important that the resulting pattern size have a particular accuracy, i.e., that the absolute dimensions of figures in that pattern be known and within a given dimensional range for each such printed or projected pattern. For example, certain pattern dimensions or linewidths, such as apertures or MOS gate electrodes, are critical during production of integrated circuits (ICs).
Betwen the mask and mask making machines and between the mask and semiconductor wafer there may typically be zero magnification error or perfect pitch with respect to well calibrated, state-of-the-art projection alignment optics equipment itself. However, dimensional variation of projected figures may be caused by a number of other factors in the printing or projecting processes, including variations in light intensity, photoresist thickness, exposure time, numerical aperture and developing time and temperature. For example, the resulting exposed portion of a photoresist pattern may have a greater linewidth than the corresponding mask slot. This may be caused by "shadowing" during printing. Subsequent IC fabrication processes, such as etching, may increase this dimensional variation by undercutting.
Several means have previously been used to calibrate or measure the dimension of such resulting patterns. However, none of these provide an absolute measure of dimensions. Measurement masks or slides of low temperature coefficient glass material may be registered with the National Bureau of Standards, but are not truly absolute in practice and are subject at at least some variation caused by thermal expansion or contraction in periods between recalibration. Recalibration of the measurement tool from such slides over periods of time is not exactly repeatable; an average of several sets of measurements is typically employed. Further, such measurement tools also leave repetition inaccuracies at any given point in time and are subject to drift or perturbation from the last previous calibration.
Another problem with prior direct measurement devices is that automatic camera-sight type machines may be unable to detect some critical dimensions directly, since they have limited resolution of such features as apertures or metal. This problem may also arise with prior concurrent distortion scale patterns which are printed onto masks, photoresists, and/or semiconductor wafers along with the desired circuitry. Such an arrangement of scale patterns is shown, for example, in U.S. Pat. No. 4,288,157, issued to Brunner. These scale patterns also do not readily allow for measurement of dimensional decreases and may require complex, expensive density detection equipment. Further, such scale patterns may not be easily and efficiently printable to show extremely fine dimensional variations.
It is therefore an object of the present invention to provide a means for ensuring the dimensions of projected or printed figures.
Another object is the provision of a means to detect and measure the dimensional variation of printed figures caused during the printing process.
A further object is to provide an absolute measure of linewidth of figures projected onto semiconductor wafers or wafer masks.
Still another object is the provision of a means for preventing calibration and drift errors in the measurement of dimensions of a series of reproduced figures.
Yet still another object is to provide a means for measuring critical dimensions in a series of repetitiously printed figures by means of relatively inexpensive electrical or optical equipment.
These and other objects of the present invention are attained in the provision of a pattern means, coded onto a mask or template, for scaling of dimensional variation of figures projected from that mask onto a lower or projected surface, such as a semiconductor wafer. This pattern means includes a plurality of individual, spaced scaling figures which are arranged such that the alignment of opposing edges of at least two of these scaling figures, which are laterally offset from each other along the axis of edge alignment on the projected surface, will represent a measure of the dimensional variation caused during the projecting or printing process. This dimensional variation may be correlated to the absolute dimension of critical features, such as linewidth, and indicia representing this correlation may be included within the pattern means along the corresponding alignment axis.
Alignment of opposing edges of scaling figures may be detected by electrical edge discriminating equipment or an optical microscope having a hairline eyepiece. Scaling figures may be arranged into columns with one member of each set of aligning scaling figures in each column. Alignment detection using an optical microscope would employ the hairline as an alignment axis and determine dimensional measurement by finding which row of scaling figures in the columns has scaling figures whose opposite edges line up on the hairline. An indicia of absolute dimension associated with that row may be directly observable under the microscope.
Since dimensional variation of figures during the printing or projecting process is usually a constant value in a given instance, only measurements along a single alignment axis are necessary in most cases. Further, the pattern means described may be used as a measurement means in formation of the mask, photoresist surface, and/or the permanent wafer pattern since it is projected along with the desired circuitry and is subject to concurrent distortion therewith. To conserve space, the individual scaling figures may be formed with a plurality of opposing edges and arranged such that each scaling figure forms part of a plurality of sets of aligning scaling figures.
Other objects, advantages, and novel features of the present invention will be readily apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.